Special Relativity. Clock A is at rest in our frame of reference and Clock B is moving at speed...

Special Relativity

Clock A is at rest in our frame of reference and Clock B is moving at speed (3/5) c relative to us. Just as Clock B passes Clock A, both read 12 am. a) When Clock A reads 5 am, what does Clock B read,

as observed in our frame?b) When Clock B reads the time found in a), what does

Clock A read as observed in Clock B’s frame?

CH 4, Sample Problem 2

A particular muon lives for a time of 2.0 μs in its rest frame. If it moves in the laboratory a distance d = 800 m from creation to decay, how fast did it move according to the lab frame?


Alpha Centauri, our nearest neighbor, is at a distance of 4 light years from us. Suppose travelers move at a speed of (4/5) c relative to Earth.According to clocks on Earth and Alpha Centauri, how long does it take the spaceship to arrive?

1. 3 years2. 4 years3. 5 years4. 8 1/3 years

Quiz Question 1

Alpha Centauri, our nearest neighbor, is at a distance of 4 light years from us. Suppose travelers move at a speed of (4/5) c relative to Earth.According to clocks on the spaceship, how long does the trip take?

1. 3 years2. 4 years3. 5 years4. 8 1/3 years

Quiz Question 2

Alpha Centauri, our nearest neighbor, is at a distance of 4 c-yrs from us. Suppose travelers move at a speed of (4/5) c relative to Earth.According to the spaceship, what is the distance to the star?

1. 2.4 c-yrs2. 4 c-yrs3. 6 2/3 c-yrs4. None of the above

Quiz Question 3

Three rules:1. Moving clocks run slow by the factor .2. Moving objects are contracted by the factor

.3. Two clocks synchronized in their own frame are

NOT synchronized in other frames. The front (leading) clock reads an earlier time (lags) the chasing clock by

where D is the rest distance between them.Notice: In rule 1, an observer with a clock finds that a moving clock

runs at a different rate by the multiplicative factor .

In rule 3, an observer finds that two moving clocks (mutually at rest), run at the same rate as one another, however, the observer has to add Δt to the reading of the leading clock to get the reading of the chasing clock.

Summary

A spaceship of rest length D = 500 m moves by us at speed v = (4/5)c. There are two clocks on the ship, at its nose and tail, synchronized with one another in the ship’s frame. We on the ground have three clocks A, B, and C, spaced at 300 m intervals, and synchronized with one another in our frame. Just as the nose of the ship reaches our clock B, all three of our clocks as well as clock N in the nose of the ship read t = 0.

a) At this time t = 0 (to us) what does the clock in the ship’s tail read?

b) How long does it take the ship’s tail to reach us at B?c) At this time, when the tail of the ship has reached us, what do

the clocks in the nose and tail read?


Sketch the spaceship of Sample Problem 1 in the ship’s frame

a) when clock B passes clock N, and b) when clock B passes clock T. In each sketch label the

readings of all five clocks, A, B, C, N, and T.


Explorers board a spaceship and proceed away from the Sun at (3/5)c. Their clocks read t=0, in agreement with the clocks of earthly observers, at the start of the journey. When the explorers’ clocks read 40 years, they receive a light message from Earth indicating that the government has fallen.a) Draw a set of three pictures in the Sun’s frame, one for each of the

three important events in the story.b) Draw a set of three pictures in the ship’s frame, one for each of the

three important events.c) In the explorers’ frame, how far are they from the Sun when they

receive the signal?d) According to observers in the explorers’ frame, what time was the

message sent?e) At what time do the stay-at-homes say the message was sent?f) According to the explorers, how far from the Sun were they when the

message was sent?g) According to the stay-at-homes, how far away was the spaceship

when the message was sent?


A bus of rest length 15.0 m is barreling along the interstate V=(4/5)c. The driver in the front and a passenger in the rear have synchronized their watches. Parked along the road are several Highway Patrol cars, with synchronized clocks in their mutual rest frame. Just as the rear of the bus passes one of the patrol cars, both the clock in the patrol car and the passenger’s watch read exactly 12:00 noon.a) When its clock reads 12:00 noon, a second patrol car happens

to be adjacent to the front of the bus. How far is the second patrol car from the first?

b) What does the bus driver’s watch read according to the second patrol car?

3. The passenger who had been sitting at the back of the speeding bus in SP 2 starts to run forward at speed (3/5)c relative to the bus. How fast is the passenger moving as measured by the parked patrol cars?


Figure 9.1 graphs how two different coordinate systems (x, y and x’, y’), one rotated relative to the other, represent the position of a point in ordinary 2D space. The figure helps us visualize the transformation Eqs. 9.3. Find an analogous graph for the Lorentz transformation between coordinates t, x and t’, x’, as given by Eqs. 9.4.


A spacetime shoot-out. Spaceships B and C, starting at the same location when each of their clocks reads zero, depart from one another with relative velocity (3/5)c. One week later according to B’s clocks, B’s captain goes berserk and fires a photon torpedo at C. Similarly, when clocks on C read one week, C’s captain goes crazy and fires a photon torpedo at B. Draw 2D spacetime diagrams of events in (a) B’s frame(b) C’s frame.Which ship gets hit first?


A particle of mass m moves with speed (3/5)c. Find its momentum if it moves (a) purely in the x direction,(b) at an angle of 45° to the x and y axes.


A particle at rest decays into a particle of mass m moving at (12/13)c and a particle of mass M moving at (5/13)c. Find m/M, the ratio of their masses.


Starship HMS Pinafore detects an alien ship approaching at (3/5)c. The aliens have just launched a 1-tonne (1000 kg) torpedo toward the Pinafore, moving at (3/5)c relative to the alien ship. The Pinafore immediately erects a shield that can stop a projectile if and only if the magnitude of the projectile’s momentum is less than 6.0 x 1011 kg m/s.(a) Find the torpedo’s momentum four-vector in the alien’s rest

frame.(b) Find the torpedo’s momentum four-vector in Pinafore’s frame.(c) Does the shield stop the torpedo?(d) Find the torpedo’s velocity relative to the Pinafore.


A photon of energy 12.0 TeV (1TeV=1012 eV) strikes a particle of mass M0 at rest. After the collision there is only a single final particle of mass M, moving at speed (12/13)c. Using eV units, find a) the momentum of the final particle,b) the mass M,c) the mass M0.


Spaceship A fires a beam of protons in the forward direction with velocity v= (4/5)c at an alien ship B fleeing directly away at velocity V=(3/5)c. Transforming the beam’s energy-momentum four-vector using the Lorentz transformation of Eqs. 11.15, find the beam’s velocity v’ in the frame of ship B.


Special Relativity. Clock A is at rest in our frame of reference and Clock B is moving at speed...

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